Sintering processes which are presently known and in use are of the following general type. An ore which is to be sintered is mixed with a solid fuel such as coal, return fines and an appropriate amount of water. The resultant mixture is pelletized or nodulized and placed on a conveyor belt. The height of the deposited layer may generally vary between approximately 30 and 70 centimeters. The mixture is delivered to an ignition hood equipped with burners which furnish hot fumes at the temperature necessary for the fusion of the ores. The mixture is ignited by means of the combustible solid fuel which is included therein. When an iron ore similar to that found in the geographic area of Lorraine, France, is to be sintered, the heat supply required for satisfactory sintering is on the order of 50 thermies per ton (th/t) of sinter (a thermie being an international unit approximately equal to 4 million B.T.U.'s). After the mixture is ignited, cold air is passed downwardly therethrough until the combustion front reaches the conveyor bars. The resultant sinter is then cooled either on a conveyor belt by means of cold air passing downwardly as in the agglomeration phase, or by means of an outside cooler after having been partly crushed and possibly screened.
There are variations to this generally described process. A portion of the solid fuel may be replaced with heat supplied in the gaseous form by hot fumes produced in a hood situated immediately after the ignition hood. The temperature of these fumes can be varied along the length of this second hood and their oxygen content must be sufficient in order to suitably burn the solid fuel of the mixture. However, even when an optimization of the temperature and the oxygen content of the hot fumes is achieved, the heat contribution of hot fumes cannot exceed 10% of the total heat contribution to the process without a major decline in the heat balance. Another variant involves predrying the mineral charge prior to ignition. However, this may be done only at a low temperature of less than 350.degree. C. or by using neutral fumes in order to prevent the solid fuel from catching fire. The aforementioned processes however entail the following disadvantages.
(a) a large heat loss occurs due to unburned fuel material in the fumes (CO, volatile substances); this represents a minimum of 15% of the heat contributions of the solid fuel, for example, 60 th/t of Lorraine ore sinter;
(b) a noticeable heat loss occurs in the sinter (from 60 to 100 th/t), a portion of which, to be sure, can be recovered in the cooling process, but which yields heat at a low level (300.degree. C. maximum);
(c) the resultant agglomerate reduction capability is far from the optimum because the use of solid fuel locally raises the sinter to a temperature which is definitely higher than the ore fusion point (1220.degree.-1230.degree. C., for example, for an iron ore from Lorraine) which leads to a loss of porsity of the material; and
(d) a major expulsion of the polluting gases of the NOx and SOx type occurs from the use of coal, together with the associated problems of corrosion.